Title: Interfacial Influence on Surface Plasmon Resonances in Quantum-Sized Nanoparticles
 

Speaker: Dr. Yugang Sun
              Center for Nanoscale Materials, Argonne National Laboratory, USA.
 

Place: A306, 1 West Building, Qujiang Campus
 

Time: 15:00, Dec. 12th, 2014

Abstract:
Nanoparticles made of noble metals such as silver and gold exhibit strong optical absorption due to their surface plasmon resonance (SPR) that corresponds to the collective oscillation of surface conduction electrons in response to the incident electromagnetic waves. As the nanoparticles are smaller than 20 nm the interfacial quantum effect may take place to influence the SPR of the nanoparticles.For example, the monodispersed silver nanoparticles synthesized via a well-defined chemical reduction process exhibit an exceptional size-dependence of SPR peak positions: as particle size decreases from 20 nm the peaks blue-shifts but then turns over near ~12 nm and strongly red-shifts.  Theoretic modeling and calculations reveal that the surface chemistry corresponding to the interactions between the capping molecules and the surface silver atoms in the nanoparticles become pronounced in determining their optical properties because the surface silver atoms represent a significant fraction of the total number of atoms in small nanoparticles.Such surface chemistry reduces the density of conduction band electrons (i.e., free electrons) in the surface layer of metal atoms, thus consequently influences the frequency-dependent dielectric constant of the metal atoms in the surface layer and the overall SPR absorption spectrum.  Another class of hybrid silver-gold nanoparticles will also be discussed.  The coupling between the SPR of the silver and gold components leads to very interesting optical signals that can be interpreted with classical plasmonics in combination with a quantum-corrected model.A comprehensive understanding of the relationship between interfacial coupling/chemistry and optical properties will be beneficial to exploit new applications of small colloidal metal nanoparticles, such as colorimetric sensing, electrochromic devices, surface enhanced spectroscopies, and photocatalysis.

This work was performed at the Center for Nanoscale Materials, a U. S. Department of Energy, Office of Science, Office of Basic Energy Sciences User Facility under Contract No. DE-AC02-06CH11357.